This thesis identifies several important features of internal gravity waves in large lakes, based on observational evidence and theoretical investigations.

In a series of field experiments in Lake Kinneret (Israel), the internal wave response to wind forcing is described as a collection of modes affected by the earth's rotation. The internal wave response was found to consist of a vertical mode one Kelvin wave, and basin-scale Poincaré waves of vertical modes one, two and three. The Kelvin wave was found to dominate the velocity field at the boundaries, with velocities in the interior dominated by the higher vertical mode Poincaré waves. The Kelvin wave is shown to exist in resonant and non-forced states with the wind, and the resonance is shown to control the seasonal evolution of the Kelvin wave.

Using an analytical model, it is shown that the dispersion relation and the total ratio of potential to kinetic energy in basin-scale internal gravity waves in large lakes is dependent on the direction of rotation relative to the earth, the aspect ratio, the Burger number (S=c/Lf) and the horizontal mode (azimuthal and radial). For the cyclonic (rotating in the same direction as the earth's rotation), lowest radial mode (a Kelvin wave for small S, and a Poincaré wave for large S), the ratio of potential to kinetic energy was always greater than unity, for all azimuthal modes. For all other radial modes (Poincaré waves for all S), both cyclonic and anti-cyclonic, the ratio is substantially less than unity, particularly as the Burger number decreases. Basin-scale Poincaré waves were found to have the same gravity/rotation balance as plane progressive Poincaré waves in a laterally unbounded system, where rotation becomes increasingly important as the Burger number decreases. The analysis suggests that horizontal transport is not always dominated by waves with large temperature signals. The total energy in basin-scale rotating waves was computed using the model, and used to determine the decay timescale of the basin-scale waves in Lake Kinneret. A methodology is proposed for determining the approximate displacement and velocity fields associated with internal waves in large lakes, based on a single measuring station.

Observations of the internal wave field also showed a substantial energy peak at frequencies just below the buoyancy frequency. The waves were of the first vertical mode, occurred in groups and resulted in isotherm displacements of up to four metres in 22 metres of water. The appearance of this high frequency internal wave energy was closely related to the strength of the wind field, and only weakly related to the phase of the basin-scale internal waves, implying a generation mechanism associated with shear instabilities in the surface mixed layer. The energy at the high frequency end of the internal wave spectrum did not come from an energy cascade from lower frequency internal waves, but was directly extracted from the wind-induced surface layer currents.

Email: antenucc@cwr.uwa.edu.au